The SAGNAC interferometer and the physical phenomena involved thereby are well known. Reference may be made for example about that to “The Fiber-Optic Gyroscope”, H. Lefèvre (Artech House, 1993).
In such an interferometer, a splitting plate or any other splitting device splits an incident wave at the input of the interferometer into two waves. The two thus-created waves are referred to as “counter-propagating waves”. They indeed propagate in opposite directions along a same closed optical path, then recombine with each other, producing interferences at the time of their recombination. The interference state between the two counter-propagating waves then depends on the relative phase difference between them. The luminous power P measured at the output of a SAGNAC interferometer is of the form: P(Δφ)=P0[1+cos(Δφ)], where Δφ is the relative phase difference between the two counter-propagating waves. Hence, the power measured at the output of the interferometer takes values between a minimum (it is then talked about “dark” fringe) and a maximum (“bright” fringe) as a function of the value of the phase difference Δφ.
It is known that some physical phenomena are liable to introduce so-called non-reciprocal phase shifts, in the counter-propagating waves, hence generating a phase difference Δφp between these waves and modifying the interference state during the recombination thereof. Hence, the measurement of this non-reciprocal phase shift Δφp allows to quantify the phenomenon that has been generated thereby.
The main physical phenomenon liable to create non-reciprocal disturbances is the SAGNAC effect produced by the rotation of the interferometer about an axis perpendicular to the plane of its closed optical path. A second effect, the FARADAY effect—or collinear magneto-optic effect—is also known for producing non-reciprocal effects of this type.
It is known that a SAGNAC interferometer can include a fiber-optic coil, which is preferably single-mode and of the polarization-maintaining type. The multiple turns of an optical fiber form a closed optical path of very long length, up to several kilometers.
A proper frequency fp of the SAGNAC interferometer is commonly defined. The proper frequency fp of a SAGNAC ring interferometer including a single-mode fiber-optic coil (silica fiber having a refractive index close to 1.5 in the operating wavelength range) of 1 kilometer long is of the order of 100 kilohertz (kHz). The extension of the coil length and hence of the optical path has for advantage to provide the interferometer with a greater sensitivity.
Moreover, it has been shown that the measurement accuracy is improved by the use of a so-called “phase cancellation” method, also called closed-loop operation, instead of a simple open-loop operation.
According to this method, an additional so-called “feedback” phase difference Δφcr is introduced by means of a phase modulator between the two counter-propagating waves, so as to compensate for the phase shift Δφp produced by the parameter measured. The sum of the two phase-shifts Δφp and Δφcr is kept at zero, which allows to make the interferometer operate with a better accuracy. The measurement of the parameter to be measured is made thanks to the use of the signal necessary to the production of the feedback phase difference Δφcr.
However, the sensitivity of the response P(Δφ) of the interferometer in the vicinity of the zero phase difference (Δφ=0) is low, because the signal measured at the output of the interferometer is a cosine-wave function of the phase difference Δφ.
It is known that it is possible to displace the operating point of the interferometer towards a point offering a greater sensitivity. It has notably been proposed to introduce an additional so-called “biasing” phase-difference modulation Δφb, by means of the phase modulator. The total phase difference Δφt between the two counter-propagating waves is then equal to the sum of the different phase differences: Δφt=Δφp+Δφcr+Δφb.
A simple-to-implement solution to perform this biasing consists in a square-wave periodic modulation at a biasing modulation frequency fb, the modulation having for example levels +π/2 and −π/2. This biasing phase-difference modulation Δφb produces at the output of the interferometer a square-wave periodic modulated electrical signal at the biasing modulation frequency fb whose amplitude is a sine-wave function of the sum of the two phase-shifts Δφp and Δφcr, in the case of a closed-loop measurement as described above. The response provided by the SAGNAC interferometer can hence be used with a greater sensitivity.
Moreover, in order to improve the stability of the measurement of a non-reciprocal parameter by means of a SAGNAC interferometer, the document EP0430747 proposes a device in which the biasing phase-difference modulation MOO introduced between the two counter-propagating waves is periodic at the frequency fb.
At each period of the modulation, the level of the phase-difference modulation Δφb(t) is hence equal to:                φ0 during the first quarter of period,        aφ0 during the second quarter of period,        −φ0 during the third quarter of period, and        −aφ0 during the fourth quarter of period.        
The values of a and φ0 are chosen so as to verify the relation: cos(φ0)=cos(aφ0).
The device according to the document EP0430747 also includes a signal processing system using the four values taken by the modulated electrical signal delivered by the interferometer during one modulation period. The signal processing system then allows to maintain constant the gain of the modulation chain so as to compensate for the slow drifts of the different components of the device (for example: variation as a function of the temperature).
To reduce the effects of the modulation chain defects on the measurement, it is known that the biasing modulation frequency fb has to be equal to the proper frequency fp of the interferometer or to one of its odd multiples.
In particular, the so-called “four states” modulation generated by the biasing described in the document EP0430747 introduces defect-bearing peaks on the modulated electrical signal measured at the output of the interferometer, these defects being eliminated when the biasing modulation frequency fb is equal to the proper frequency fp of the SAGNAC interferometer, or to one of its odd multiples.
Moreover, the number of these peaks increases with the biasing modulation frequency fb.
To reduce the response time of the interferometer and to ensure that the feedback loop of measurement does not break in case of a rapid variation of the parameter to be measured, the biasing modulation frequency fb is increased. However, the measurement accuracy is hence substantially degraded due to a greater number of peaks in the signal detected.